This project will analyze the feasibility of using the Calypso electromagnetic localization system to track tumors that move with breathing. Such a capability will allow the system to operate through a control loop to keep an external radiation therapy beam on target while the patient breathes. The Calypso system uses small electromagnetic coils that can be implanted in a tumor and then located remotely, with millimeter-scale accuracy, by a transmitter/receiver system positioned above the patient. This technology was initially developed to replace image-based localization of the tumor site for patients undergoing external-beam radiotherapy, and to provide continuous tracking during the course of irradiation. Because the system can sample the tumor position at a rate of ten measurements per second, it is capable in principle of real-time tumor tracking. The feasibility of applying real-time tracking to lung radiotherapy will be analyzed by characterizing the accuracy and time response of the Calypso measurement system, designing and characterizing a control loop to translate the Calypso measurement into beam control data while accommodating system lag time, and then assessing the complete control process performance against measured clinical data for tumor motion during free breathing. Electromagnetic tumor tracking will supplement and potentially replace the intensive radiological imaging that is presently needed to locate moving tumors during treatment. This will result in a significant reduction in non-therapeutic radiation dose to the patient while simplifying the tumor localization process. The augmented Calypso targeting system will be designed for integration into existing radiation therapy systems to provide enhanced beam targeting capability. [unreadable] [unreadable]